Ireland have suffered a blow ahead of their match with Scotland on Sunday following the news that Chris Henry will sit out the rest of the Six Nations with a knee injury.

Henry suffered a cartilage tear to his right knee during Ulster's 26-3 RaboDirect PRO12 victory over Zebre on Friday night and must undergo an operation. An Irish Rugby Football Union statement said that he will be out "for approximately four weeks."

The Ulster flanker featured on the bench for Ireland's first two matches and got game time against both Wales and France. The news of his setback will add to Ireland's woes with their injury list ever increasing.

Centre Gordon D'Arcy and wing Simon Zebo have been lost for the remainder of the competition, while fly-half Jonathan Sexton and lock Mike McCarthy will definitely miss Sunday's clash with Scotland.

Concerns continue to hang over the fitness of centre Brian O'Driscoll and second row Donnacha Ryan, while prop Cian Healy has been ruled out of the games against Scotland and France through suspension. Ireland entered the tournament without lock Paul O'Connell and wing Tommy Bowe, who will not return in time, while flanker Stephen Ferris' involvement is in doubt.

Munster back row Tommy O'Donnell has been called into the squad as cover for Henry, an openside who started the Wales and England matches on the bench. Connacht centre Dave McSharry is also unavailable due to a groin injury, so Leinster's Ian Madigan has been called up.